1. Field of the Invention
The present invention relates to a receiver, and more particularly to a superheterodyne receiver designed to receive the radio signals output by a transmitter and representing digital data.
2. Description of the Related Art
A transmitter has been put to practically use, which can transmit radio signals representing digital data, over long distances. The radio signals have been generated in the following process. First, the digital data is converted into a serial frame consisting of bits; then, the frame is changed into radio signals, each at a high level corresponding to a "1" bit or a low level corresponding to a "0" bit. The radio signals are subjected to frequency-shift keying (FSK), and then transmitted from the transmitter. A receiver receives and converts the radio signals into the digital data.
The receiver comprises an antenna, a high-frequency amplifier, a local oscillator, a frequency conversion circuit, an IF (intermediate-frequency) amplifier, and detector circuit. The high-frequency amplifier amplifies the signals the antenna have received, and supplies these signals to the frequency conversion circuit. The local oscillator produces and supplies signals to the frequency conversion circuit. The frequency conversion circuit mixes the signals supplied from the high-frequency amplifier with those output by the local oscillator, thus generating IF signals. The IF amplifier amplifies the IF signals. The detector circuit detects and converts the amplified IF signals into digital data.
The receiver further comprises a display which displays data showing whether or not the receiver is receiving radio signals. Reading the this data, the operator knows whether or not the transmitter is receiving signals. The display must be driven even while no signals are being input to the receiver. Hence, it is necessary to supply power to the receiver from a power source such as a battery, no matter whether or not signals are being input to the receiver.